This application claims the benefit of Korean patent application No. 2000-51295, filed Aug. 31, 2000 in Korea, which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to an optical detecting sensor, and more particularly, to a thin film transistor (TFT) type optical detecting sensor.
2. Discussion of the Prior Art
In general, optical detecting sensors are used in facsimile and digital copying machines, and in fingerprint recognition systems as image readers. The optical detecting sensor stores electric charges according to an intensity of light reflected from a detecting subject, and then outputs the electric charges via a drive circuit. Recently, TFT type optical detecting sensors have been suggested in optical detecting systems such that the TFT changes its electrical characteristics in response to incident light.
An inverted staggered type TFT has been selected for typical TFT type optical detecting sensors because of its simple structure and superior quality. The inverted staggered type TFT has been classified into at least two categories: a back channel etch type TFT and an etch stopper type TFT.
A typical TFT type optical sensor will include a light source for generating light, a window for introducing the light to a subject for detection, a sensor TFT, a storage capacitor, and a switch TFT. The sensor TFT generates an optical current according to an intensity of the light reflected from the subject for detection, and the storage capacitor receives the optical current and stores electric charges of the optical current as data. Then, the switch TFT transmits the electric charges according to a control signal generated from an exterior circuit, to transfer the data to a main system.
FIG. 1 shows a conventional TFT type optical sensor including an array substrate 1 and a back light unit 2 disposed under the array substrate 1. The array substrate 1 detects a subject, stores data relating to the subject, and transmits the data to a main system (not shown) such as a fingerprint recognition system, for example. The back light unit 2 generates light for the array substrate 1. As shown in FIG. 2, the array substrate 1 includes a plurality of unit pixels xe2x80x9cPxe2x80x9d each including a sensor TFT xe2x80x9cT1,xe2x80x9d a storage capacitor xe2x80x9cC,xe2x80x9d and a switch TFT xe2x80x9cT2.xe2x80x9d The sensor TFT xe2x80x9cT1xe2x80x9d and the switch TFT xe2x80x9cT2xe2x80x9d are both conventionally formed of the back channel etch type TFT, for example.
FIGS. 2 and 3 show the unit pixel xe2x80x9cPxe2x80x9d to include a sensor gate line 21, a sensor data line 61, a switch gate line 25, and a switch data line 65. The sensor gate line 21 and the sensor data line 61 cross with each other, and the switch gate line 25 and the switch data line 65 are spaced apart from the sensor gate line 21 and the sensor data line 61, respectively. The unit pixel xe2x80x9cPxe2x80x9d is divided into a photo-sensing region xe2x80x9cA,xe2x80x9d a storing region xe2x80x9cB,xe2x80x9d and a switching region xe2x80x9cC,xe2x80x9d all of which are formed on a transparent substrate 10. A sensor gate electrode 22, a first capacitor electrode 24, a switch gate electrode 26 are formed in the photo-sensing region xe2x80x9cA,xe2x80x9d the storing region xe2x80x9cB,xe2x80x9d and the switching region xe2x80x9cC,xe2x80x9d respectively. The sensor gate electrode 22 and the switch gate electrode 26 integrally protrude from the sensor gate line 21 and the switch gate line 25, respectively. Alternatively, parts of the sensor gate line 21 and the switch gate line 25 may not protrude, but may be used as the sensor gate electrode 22 and the switch gate electrode 26, respectively. The first capacitor electrode 24 integrally protrudes from the sensor gate line 21.
In FIG. 3, a first insulating layer 30 covers the sensor electrode 22, the first capacitor electrode 24, and the switch gate electrode 26. On the first insulating layer 30, a sensor silicon layer 41 and a switch silicon layer 42 are formed in the sensing region xe2x80x9cAxe2x80x9d and the switching region xe2x80x9cB,xe2x80x9d respectively. A sensor ohmic contact layer 52 and a switch ohmic contact layer 54 are formed on the sensor silicon layer 41 and the switch silicon layer 42, respectively.
A sensor source electrode 62 and a sensor drain electrode 63 are formed over the sensor silicon layer 41, and a switch source electrode 66 and a switch drain electrode 67 are formed over the switch silicon layer 42. A first capacitor electrode 24 integrally protrudes from the sensor gate line 21 toward the unit pixel region xe2x80x9cP.xe2x80x9d The sensor source electrode 62 is connected with the sensor data line 61, and the sensor drain electrode 63 is spaced apart from the sensor source electrode 62 with the sensor gate electrode 22 centered therebetween. The switch source electrode 66 is connected with the switch data line 65, and the switch drain electrode 67 is spaced apart from the switch source electrode 65 with the switch gate electrode 26 centered therebetween. A second capacitor electrode 64 is formed between the switch drain electrode 67 and the sensor drain electrode 63 and is interconnecting therewith. The second capacitor electrode 64 overlaps the first capacitor electrode 24.
A second insulating layer 70 covers the sensor source electrode 62, the sensor drain electrode 63, the second capacitor electrode 64, the switch source electrode 66, and the switch drain electrode 67. On the second insulating layer 70, a shielding pattern 80 made of an opaque material is formed over the switch silicon layer 42.
For the above-described optical detecting sensor according to the prior art, the sensor silicon layer 41 preferably has a thickness larger than 3000 xc3x85 (angstrom) to provide high efficiency. Accordingly, since the switch TFT xe2x80x9cT2xe2x80x9d is formed by the same fabrication process of forming the sensor TFT xe2x80x9cT1,xe2x80x9d the thickness of the switch silicon layer 42 is also preferably larger than 3000 xc3x85. Although the preferably large thickness of the silicon layer provides for high efficiency of the sensor TFT xe2x80x9cT1,xe2x80x9d the large thickness increases off current of the switch TFT xe2x80x9cT2xe2x80x9d, thereby causing noise.
Accordingly, the present invention is directed to a TFT type optical detecting sensor that substantially obviates one or more of problems due to limitations and disadvantages of the prior art.
An object of the present invention is to provide an improved TFT type optical sensor wherein silicon layers of sensor TFT and switch TFT have different thicknesses to achieve high efficiency of the sensor TFT and to decrease off current of the switch TFT.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, a thin film transistor optical detecting sensor including an array substrate comprising a transparent substrate, a plurality of sensor thin film transistors disposed on the transparent substrate, each having a first silicon layer of a first thickness, a plurality of storage capacitors, each connected with a corresponding one of the plurality of sensor thin film transistors, storing charges of an optical current, and a plurality of switch thin film transistors, each having a second silicon layer of a second thickness less than the first thickness.
In another aspect, a method of fabricating a thin film transistor optical sensor includes steps of forming a first metal layer on a substrate, the first metal layer includes a sensor gate electrode, a switch gate electrode, and a first capacitor electrode, forming a first insulating layer on the first metal layer, forming an amorphous silicon layer and an etch stop layer on the first insulating layer, the etch stop layer disposed over the sensor gate electrode, forming a doped amorphous silicon layer to cover the amorphous silicon layer and the etch stop layer, forming a sensor silicon layer, a switch silicon layer, a sensor ohmic contact layer, and a switch ohmic contact layer from the doped amorphous silicon layer and the amorphous silicon layer, and forming a second metal layer to include a sensor source electrode, a sensor drain electrode, a switch source electrode, and a switch drain electrode.
In another aspect, an array substrate for a thin film transistor optical detecting sensor, the array substrate includes a transparent substrate, a plurality of sensor thin film transistors each having a sensor silicon layer of a first thickness, each sensor thin film transistor generating an optical current in response to light reflected from a detection subject, a plurality of storage capacitors, each connected with a corresponding one of the plurality of sensor thin film transistors, storing charges of the optical current, and a plurality of switch thin film transistors, each having a switch silicon layer of a second thickness less than the first thickness, wherein each switch thin film transistor is electrically connected with a corresponding one of the plurality of storage capacitors and selectively outputs the charges stored in the storage capacitor.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.